1. Field of the Invention
This invention relates to the field of forming three-dimensional structures in an etchable crystalline substrate material, in particular to the formation of perfect exterior corners intersected by features formed in the material using micromachining.
2. Description of Prior Art
The ability to fabricate precise, three-dimensional structures in an etchable substrate material, particularly in silicon, relies to a large degree on the ability to perform so-called anisotropic etching of the single crystal silicon substrate. Anisotropic etches have the property of etching certain crystallographic planes of silicon much more rapidly than others by using some etchants such as mixtures of KOH and water or mixtures of ethylene diamine, pyrocatechol, and water. All of these etches etch the {111} silicon plane much more slowly than the other low order {100} or {110} planes.
The two most common surface orientations of silicon wafers are (100) and (111). Since the etch rate of {111} planes is so low, (100) silicon is the preferred orientation for device fabrication. In (100) silicon, a flat is provided on each wafer along a &lt;110&gt; direction. In this orientation, the {111} planes intersect the (100) surface parallel and perpendicular to the wafer flat, at an angle of 54.74.degree. with respect to the surface as shown in FIG. 1A in cross section.
If an opening in an etch mask is opened to form a rectangle, an anisotropic etchant will etch down exposing the {111} planes, to form a V-grove as shown in FIG. 1A and 1B in a plan view.
If a window in an etch mask is formed which is more complicated in shape than a rectangle, any convex protrusion will etch back to the "farthest" {111} plane given enough time, as shown in FIGS. 2A and 2B.
If one wants a convex corner, for example, at point A shown in FIG. 2B, use this exact L-shape as the etch mask, and put the substrate in KOH and water, the corner is etched back as shown in FIG. 3A and the protruding mask feature breaks off. So if it is desirable to have a convex corner which is not etched back, a so-called corner compensation technique may be used, in which an additional mask feature is added to the corner, which etches back just the right amount to leave the desired three-dimension feature at the exterior corner. One such corner compensation pattern is to add a square protrusion at the exterior corner as shown in FIG. 3B. As the etch proceeds downward, this additional feature etches back at the correct rate to leave the desired corner feature. This type of corner compensation has been long known.
However, with such corner compensation, the perfect intersection can only happen at a particular depth, which is the depth for which the correct corner compensation has been provided. In other words, the depth has to exactly match this additional mask feature, and the moment of perfect intersection (depth) is very short and hard to control. Furthermore, one may sometimes want a groove A to be very shallow and intersect a much deeper groove B as shown in FIG. 3C. For example, groove A is 10 microns wide and about 7 microns deep, and groove B can be 100 microns deep and 140 microns wide. In such a case, with the corner compensation scheme one would have to add a very large corner compensator feature to the corner, but normally there is no room for this. The corner compensation therefore cannot be used in some circumstances.
Another related problematic situation is when it is desired to form a through hole in a wafer by etching simultaneously from the back and front sides of the wafer, simultaneously as shown in FIG. 4A. Just after the two grooves 1 and 2 meet, the features begin etching back from the point of intersection of the {111} planes at a rapid rate. If left long enough, the sidewalls etch back to meet the other set of {111} planes forming a large cavity in the silicon, as shown in FIG. 4B; this is undesirable if a hole having a well defined diameter is needed.